1. Field of the Invention
The present invention relates to a PZT piezoelectric material, a method for producing the PZT piezoelectric material, as well as a piezoelectric device and a liquid discharge device employing the PZT piezoelectric material.
2. Description of the Related Art
Piezoelectric devices, which include a piezoelectric material that expands or contracts when the intensity of an applied electric field is increased or decreased, and an electrode for applying the electric field to the piezoelectric material, are used in applications, such as piezoelectric actuators provided in inkjet recording heads. As piezoelectric materials, PZT (lead zirconium titanate) and substitution systems of PZT, which has a part of the A-site and/or B-site thereof being substituted with a different element, have been known. PZT and the substitution systems thereof are herein collectively referred to as the “PZT system” or “PZT”.
It is known that PZT doped with a donor ion which has a higher valence than a valence of a substituted ion has higher piezoelectric performance than that of the intrinsic PZT. Examples of a donor ion that substitutes Zr4+ and/or Ti4+ at the B-site include V5+, Nb5+, Ta5+, Sb5+, Mo6+ and W6+. A PZT perovskite oxide with a part of the B-site thereof substituted with a different element M is represented by general formula (P) below:Pba(Zrx, Tiy, Mb−x−y)bOc   (P)(wherein M represents one or two or more B-site elements; wherein 0<x<b, 0<y<b, 0≦b−x−y; and wherein a molar ratio a:b:c is 1:1:3 as a standard; however, the molar ratio may be varied from the standard molar ratio within a range where a perovskite structure is obtained.)
In “Measurement of transverse piezoelectric properties of PZT thin films”, I. Kanno et al., Sensors and Actuators A, Vol. 107, pp. 68-74, 2003 (which is hereinafter referred to as “non-patent document 1”), bipolar polarization-to-electric field characteristics of a c-axis oriented PZT film having tetragonal crystal structure are evaluated. FIG. 8A shows the polarization-to-electric field characteristics of the PZT film disclosed in FIG. 2 of non-patent document 1. In this film, the direction of the axis of spontaneous polarization coincides with the direction of the applied electric field. Thus, there occurs only 180° domain switching in the film and no 90° domain rotation. The polarization-to-electric field characteristics of this film have good cornered curves, that is, there are sharp polarization changes around coercive electric fields Ec1 and Ec2. The polarization changes around the coercive electric fields Ec1 and Ec2 occur due to the 180° domain switching.
In typical piezoelectric materials, non-180° domain rotation, such as the 90° domain rotation, occurs, and therefore the polarization changes around the coercive electric fields Ec1 and Ec2 are less sharp. Therefore, electric field-to-deformation characteristics thereof exhibits hysteresis. Further, in typical piezoelectric materials, the bipolar polarization-to-electric field curve is substantially point-symmetric with respect to the origin. Therefore, the absolute value of the coercive electric field Ec1 at the negative side of the electric field of the bipolar polarization-to-electric field curve substantially coincides with the coercive electric field Ec2 at the positive side of the electric field (|Ec1|≈Ec2). FIG. 8B schematically shows a polarization-to-electric field curve of a typical piezoelectric material where non-180° domain rotation occurs.
Common piezoelectric materials are typically driven in a unipolar driving mode, i.e., within a range from the electric field of 0 to an electric field Emax where the bipolar electric field-to-deformation curve exhibits the maximum displacement at the positive side of the electric field, or within a range from the electric field of 0 to an electric field Emin where the bipolar electric field-to-deformation curve exhibits the minimum displacement at the negative side of the electric field.
In the common piezoelectric materials having the polarization-to-electric field characteristics shown in FIG. 8B, the two coercive electric fields Ec1 and Ec2 have different polarities. Therefore, the polarization switching occur during both of the unipolar driving with the positive voltage (driving within the range from the electric field of 0 to the electric field Emax where the bipolar electric field-to-deformation curve exhibits the maximum displacement at the positive side of the electric field) and the unipolar driving with the negative voltage (driving within the range from the electric field of 0 to the electric field Emin where the bipolar electric field-to-deformation curve exhibits the minimum displacement at the negative side of the electric field).
The polarization switching causes loss in the piezoelectric deformation, and thus power consumption for obtaining a desired piezoelectric deformation is increased. Therefore, it is desirable that no polarization switching occurs during the unipolar driving.
Japanese Unexamined Patent Publication No. 2003-243741 (which is hereinafter referred to as “patent document 1”) proposes a piezoelectric film having asymmetric polarization-to-electric field hysteresis characteristics, where two coercive electric field points in the hysteresis characteristics are at the same polarity of the electric field of the polarization-to-electric field hysteresis characteristics curve (see claim 1). Patent document 1 discloses that the piezoelectric film having the above-described polarization-to-electric field hysteresis characteristics is provided by changing, for example, the amount of Zr in the film thickness direction (see claim 3).